The currently known semiconductor apparatuses for detecting energy and point of incidence of electromagnetic radiations are of the following types: the so-called "Charge-Coupled Devices" (CCD), the so-called "Drift Chambers" or "Silicon Drift Detectors" (SDD), the so-called "Microstrip Detectors" and the so-called "Pixel Detectors".
CCDs essentially include a chip made of a semiconductor, typically silicon, in which a plurality of potential energy wells for the electrons located in one or more successions at predetermined distances are generated by a corresponding plurality of electrodes. The incident radiation generates electron-hole couples in the semiconductor. Holes are immediately collected by a suitable static biasing electric field generated in the semiconductor, while electrons are confined to the potential energy well which is closer to the point of incidence of the ionizing radiation. The potential energy wells are translated by timed signals along the semiconductor to one or more collector electrodes where the electrons generated by the incident radiation are collected and directed to an amplification chain.
The detection of the point of incidence of the ionizing radiation is accomplished in such apparatuses by counting the number of timed pulse signals needed to shift a determined potential well to the collector electrode. A bidimensional measurement of the point of incidence of the ionizing radiation can be made by providing a bidimensional arrangement of potential wells and a plurality of collector electrodes.
In radioastronomy, CCD apparatuses for X ray detection with high resolution in terms of energy due to the low output capacitance of the collector electrode have been developed. The resolution in the detection of the point of incidence depends on the mutual distance between the electrodes which generate the potential energy wells (dimension of the pixels).
The drawback of such apparatuses, however, is given by the need to generate such timing signals. The frequency of such signals can be limited by the need to have a sufficient translation efficiency during the movement of the potential energy wells, the time necessary for processing the signal associated to the electrons in each potential well arriving at the collector electrode, or the allowed power consumption. For example, in the spectroscopic measurements made in astronomy, the frequency of the timing signals is limited to about 100 kHz by the signal processing time. SDD detectors also include a chip made of a semiconductor, typically silicon, in which there are provided a succession of field electrodes (so-called "field strips") on both surfaces of the semiconductor chip and one or more electrodes for collecting the signal charges only on one surface. Said field strips which are biased by applying voltages that increase the magnitude with the distance from the collector electrodes, generate a static electric field (so-called electric drift field). The incident radiation generates electron-hole couples, the holes being immediately collected as for CCDs by the field strips which are closer to the source thereof (point of incidence), and the electrons drifting in parallel to the surfaces of the chip towards the collector electrodes to which the signal amplification chain is connected because of the electric drift field.
In such detectors the speed of translation of the electrons generated in the semiconductor towards the collector electrode may be greater than that in CCDs. Actually, such a speed is generally proportional to the applied electric drift field because of the lack of the above-mentioned typical limitations of CCDs concerning the charge transfer efficiency and the allowed consumption. Trials have shown, for example, that electric drift fields in the order of 200-1000 V/cm inducing drift speeds between 3 and 14 .mu.m/ns can be applied.
A drawback of such a type of detectors consists in that a reference signal which is synchronous with the radiation arrival time should be generated in order to detect the point of incidence of the ionizing radiation. This further complicates the detector design and the acquisition electronics.